Thermosiphon User Guide

7/17/2019 Thermosiphon User Guide

http://slidepdf.com/reader/full/thermosiphon-user-guide 1/8

Design and Simulate a Vertical Thermosiphon Reboiler Using

Aspen Shell & Tube Exchanger

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Outline Design Guide

Jim McNaught, Consultant, TUV NEL

Tom Ralston, Product Management, Aspen Technology, Inc.



Contents

Design of a Vertical Thermosiphon Reboiler ................................ .............................................. ...........1

Introduction ............... ................................................ ........................................ ............................1

Problem Description ................................................. ............................................... .......................2

Steps ........................................ ............................................... ......................................... ..............2

Start .............................................. ............................................... ......................................... .....3

Problem Definition ......................................................... ....................................... ......................3

Process Data ........................................ ................................................ .......................................3

Physical Properties ........ ................................................. ....................................... ......................3

Exchanger Geometry ................................ ........................................ ...........................................3

Thermosiphon Details....................................................................... ...........................................3

Nozzle Information ...................................... ................................................ ................................4

Save and Run .................................................................... ............................................... ...........4

Review of Results ..................................................................... ............................................... ....4

Simulating Vertical Thermosiphon Reboilers ............................................... .........................................4

Problem Description: ................................................ ............................................... .......................4

Steps ........................................ ............................................... ......................................... ..............4

Start-up ........................................ ............................................... ......................................... ......4

Thermosiphon Details................................................................ .............................................. ....5

Save and Run .................................................................... ............................................... ...........5

Review of Results ..................................................................... ............................................... ....5

Additional Modelling Options .............................................. ................................................ ................5

Part Load Operation..................................................................... ............................................... ....5

Clean Start Up Operation ................... ............................................. ........................................... .....6

Additional Resources ..... ............................................... ............................................ ..........................6



Design & Simulate a Vertical Thermosiphon Reboiler

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1

Design of a Vertical Thermosiphon Reboiler

Introduction

Thermosiphon reboilers are widely used in refining and chemical processing. Properly designed, they

provide an elegant solution to vaporize lighter fractions from the bottom of a distillation column andreturn the two-phase stream at an appropriate separation stage. The use of the thermosiphon effect to

circulate the fluids provides a design with lower capital and maintenance cost than a pumped system

with a kettle or other type of forced flow reboiler.

In designing a thermosiphon reboiler, gravitational head is very important and hence the relative vertical

locations of the liqud level within the column to the inlet of the heat transfer section of the exchanger.

Also of critical importance is the relative height of the return to the column. These heights are often

given relative the ground level but any data can be used to specify the relative heights, as illustrated in

Figure 1 from Help in the Aspen Shell&Tube Exchanger Program.

Figure 1: The As pen EDR Help Menu

Both horizontal shellside and vertical tubeside reboilers configurations are widely used. For very large

capacities, such as in an atmospheric crude distil lation, horizontal shellside units are prevalent. In many

lower capacity applications vertical tubeside units are often preferred as they consume le ss plot space.

It is normal to design the thermosiphon for the required circulation rate, outlet vapor fraction , andthermal duty. This can be done initially without detailing the inlet pipework bringing column bottoms to

the unit, or without detailing the return pipework. Once the exchanger itself has been sized on a

preliminary basis, the performance can be evaluated with pipework specified in detail. The simulated

circulation rate and outlet vapor fraction can be checked against the design requirement and the

stability of circulation verified.




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The guide which follows illustrates both initial design and subsequent simulation of operation with full

specification of connecting pipework.

The guide also outlines some typical trouble-shooting around part-load operation and clean start-up

with potential instability. When operated outside stability limits flow and pressure on the cold side of athermosiphon reboiler will vary rapidly in a oscillatory manner. The performance of the unit will be

much less than in stable operation. It is important to avoid instabil ity which will have a major impact on

throughput of the distillation column and on product quality.

Problem Description

The task is to design a Vertical Thermosiphon Reboiler for the following duty:

Column Pressure 5.13 bar (abs)

Cold stream

Fluid: Hydrocarbon mixtureFlowrate 90000 kg/h

Outlet vapour mass fraction (quality) 0.4

Fouling resistance 0.0001 m2K/W

Duty 3300 kW

Hot stream

Fluid Steam

Inlet pressure 3.5 bar (abs)

Allowable pressure drop 0.08 bar

Fouling resistance 0.00009 m2K/W

Inlet vapour mass fraction 1.0Outlet vapour mass fraction 0.0

Cold stream composition (mass fraction)

n-Pentane 0.72

Benzene 0.28

The available height from the liquid surface in the column to the bottom tube plate of the exchanger is

4000 mm.

A tube outside diameter 25.4 mm and thickness 2.11 mm is selected. The tube material is carbon steel.

The tubeside inlet nozzle is axial with a Cone type front cover, and there is a pla in tubeside outlet nozzleat the outlet header.

Steps

The steps described below lead to the design of the reboiler alone. Piping details will be added when a

Thermosiphon Simulation run is set up in Example 2.




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Start

Start the Aspen Exchanger Design and Rating User Interface and select a new Shell&Tube Exchanger

case. Change the Units of Measure to SI.

Problem Definition Open the Application Options input form

Specify the hot stream on the shellside, set the Application to Vaporization and set the Vaporizer

type to Thermosiphon

Process Data

Open the Process Data input form

Enter the cold stream flowrate, column pressure, inlet vapour mass fraction (zero), outlet vapour

mass fraction and fouling. Also enter the hot stream inlet pressure, allowable pressure drop, inlet

and outlet vapour fraction and fouling resistance. You do not need to enter hot stream

temperatures in this example because Shell&Tube will calculate them from the pressure and vapour

mass fractions

Physical Properties

Open the Hot Side Composition input form and specify Water as a single component from the B-JAC

Databank

Open the Hot Side Properties input form and click on the “Get Properties” button

Open the Cold Side Composition input form and select the Aspen Properties physical property

package

Select the two cold side components from the databank and enter the compositions (mass

fractions).

Move to the Property Methods tab and select Peng-Robinson/LK.

Open the Cold Stream Properties input form and click the Get Properties button. Examine the

calculated properties and check that the temperature range includes the bubble point. You shouldhave data at two pressures.

Exchanger Geometry

Open the Geometry Summary input form

Specify a vertical TEMA AEL exchanger

Set the Front Cover type to a Cone in the Shell/Heads/Flanges/Tubesheet input form

Enter the tube outside diameter and wall thickness

Thermosiphon Details

Open the Thermosiphon Piping form

Set the height of the column liquid level to 4000 mm and the height of the heat transfer region inletto 0.0. Set the height of the return line to the column to 4200 mm. The “liquid head” that drives

the circulation around the reboiler is determined by the height difference between the liquid level

and the exchanger height and the inlet pipe liquid density

Pipework pressure loss estimates, for both inlet and outlet pipework, are expressed as a percentage

of the inlet pipework static head, i.e. 'Column Liquid Level elevation minus Exchanger Heat Transfer

Region Inlet elevation' at the inlet fluid density which is assumed to be constant.




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In Design mode the details of the inlet and outlet piping might not be available and it is often useful

to select the option to express the pipework pressure loss as a percentage of the available liquid

head. The defaults of 25% and 10% respectively for the inlet and outlet pipework represent typical

values

Nozzle Information

Open the Nozzles input form

Select the Tube Side Nozzles tab and set the Tube Side Inlet Nozzle Orientation to Axial. You do not

need to enter any other data as Shell&Tube will calculate the nozzle diameters using the allowable

pressure drops.

Save and Run

Save the case with filename Reboiler Design Case.edr

Run the Design Case and save it.

Review of Results

Check that the cold stream pressure drop is mostly utilized. If not, this means that when thereboiler is operated in natural circulation, the flowrate may be much greater than the design

flowrate. To solve this problem you would probably have to increase the allowable pressure drop

on the shellside.

Check that the area ratio of the selected design is close to 1.0. If i t is greater than about 1.2, this

means that the reboiler is significantly over-sized. Again, you may have to increase the shellside

allowable pressure drop.

Simulating Vertical Thermosiphon Reboilers

Problem Description:

The task is to simulate the performance of the reboiler that was designed in the above example. The

simulation will include details of the pipework to and from the column. The main purpose of the run is

to check that required cold stream flowrate of 90,000 kg/h and the required heat load of 3,300 kW are

achieved when the reboiler is connected to the column and the flow is driven by natural circulation.

In Thermosiphon Simulation mode the user specifies the hot stream conditions and the program

iteratively calculates the reboiler circulation rate and heat load determined by the liquid head, pressure

losses around the system and the local heat transfer rates.

The approach taken is to create a Simulation case from a design run and then convert it to a

Thermosiphon run.

Steps

Start-up

Open the file Reboiler Design Case.edr and run it. Select Run, Update file from Geometry –

Shell&Tube from the main menu bar at the top of the screen to convert the Design run to a




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Checking run. Go to the Application Options input form and change from Checking to Simulation

mode

Save the file as Reboiler Simulation Case.edr

Thermosiphon Details Open the Thermosiphon Piping input form

In the Design run the inlet and outlet pressure losses were estimated as a percentage of the

available liquid head. In Simulation it is likely that at least preliminary details of the piping

configuration wil l be available, so set the Pipework Loss Calculation to From PipeWork. It is

important for an accurate Simulation that the pipework details are entered

Go to the Inlet Piping Elements form and click on the first “Inlet Circuit Element” box, when a drop -

down list box wil l appear. Select a pipe element and estimate 4,000 mm total length. Then specify 2

bends in the second column. You can enter the inlet piping elements in any order because the flow

is always in the l iquid phase. Note that the program obtains a default pipe diameter from the nozzle

diameter.

Go to the Outlet Piping Elements form and estimate 2000 mm horizontal pipe for the outlet. Since

the channel outlet is a plain nozzle, there is only a single horizontal pipe length between the reboiler

and the column.

Save and Run

Run the data set and save it.

Review of Results

Check the output to see whether the design tubeside flowrate and heat duty have been achieved

Revise and adjust the design as necessary. You might have to insert a resistance, usually a valve,

into the inlet piping to reduce the cold stream flowrate and increase the outlet vapour mass fraction

to the design value of 0.4. Do this by adding a General Element to the Inlet Piping Elements. Insert

an estimate of the number of velocity heads lost in the valve and use trial and error to adjust this

value. If you find that, when you achieve the outlet vapour fraction of 0.4 the tubeside flowrate is

greater than the design value, this is because the design run generated an over-sized exchanger. In

practice one way to reduce the heat load to the design value would be to reduce the steam

pressure. Some further iteration on the inlet circuit resistance may then be required

When you have achieved a Simulation run that matches the design conditions, review any warning

messages carefully and check the stabili ty analysis (under Flow Analysis - Thermosiphons). View the

incremental output in Analysis along Tubes to check the two-phase flow pattern at the outlet of thetubes (ideally it should be Annular).

Additional Modeling Options

Part Load Operation

The reboiler is to be controlled for part-load operation by flooding the shell by condensate. Investigate

the performance of the reboiler when part of the tube length on the shellside is flooded by condensate.




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Note that you cannot enter this directly from input, but you can use Shell&Tube to specify the

condensate outlet temperature. NOTE: Since the hot stream flowrate is defaulted, Shell&Tube will

calculate the steam flowrate corresponding the heat load and the specified inlet and outlet conditions.

You can use the Analysis along Shell output form to see the fraction of the tube length flooded, i.e.where the shell temperature is below the saturation temperature.

Clean Start Up Operation

Start from the Simulation case and investigate the performance of the reboiler when the unit is started

up for the first time, i.e. it is in the clean condition and there is no valve in the inlet pipework. You will

find that cold stream flowrate and the heat load are much higher than the design condition. Dynamic

instabil ity may be predicted as a result of the increased pressure drop in the outlet piping, in turn

caused by the increased outlet vapour fraction. The performance could be reduced to the stable design

condition by reducing the steam pressure.

Additional Resources

Public Website

www.aspentech.com/products/aspen-edr.aspx

Support Website

www.support.aspentech.com

http://www.aspentech.com/products/aspen-edr.aspx

http://www.support.aspentech.com/

http://www.support.aspentech.com/

http://www.aspentech.com/products/aspen-edr.aspx

Documents

Thermosiphon User Guide